1. Field of the Invention
The present invention relates to a display apparatus for displaying images on the basis of picture signals and, more particularly, to a display apparatus for projecting light from a light source onto a screen via a modulating device.
2. Description of the Related Art
A plasma display panel and a modulating device are presently attracting attention as compact, light-weight flat display apparatuses which can replace CRT display apparatuses. These flat display apparatuses are roughly classified into two categories; i.e., a self-luminescing type display apparatus which emits light by itself in a display operation and a transmittance control type display apparatus which controls the transmittance of incident light from a separate light source in a display operation. For example, a plasma display panel belongs to the self-luminescing type, and a modulating device belongs to the transmittance control type. In particular, the modulating device is considered as the most promising display apparatus of the next generation, so the research and development of the modulating device are being done in various application fields.
As represented by a twisted nematic type modulating device introduced in Liquid Crystal Device Handbook, a general modulating device is characterized in that light which is linearly polarized by a polarizing plate is input to a liquid crystal layer having birefringence or optical rotatory power. Unfortunately, modulating devices of this sort have the drawback that the quantity of light obtained from the light source is reduced to about one half when the light passes through the polarizing plate.
Recently, a modulating device requiring no such polarizing plate as discussed above has been developed. This modulating device has either a polymer dispersion type liquid crystal layer in which a liquid crystal material is contained in a polymer resin, or a fine-particle dispersion type liquid crystal layer in which fine particles are contained in a liquid crystal material, between a pair of transparent electrode substrates, or between a transparent substrate and a reflective electrode substrate. This modulating device functions as a scattering type modulating device which modulates the spatial propagation direction of incident rays on the liquid crystal layer. In this device the use efficiency of the light from the light source is improved in comparison with that of an apparatus using a polarizing plate.
A modulating device having the polymer dispersion type liquid crystal layer is set in a milky white, light-scattering state, in which incident light rays are scattered, in a pixel region between electrodes applied with no voltage, and in a transparent, light-transmitting state, in which incident light rays are not easily scattered, in a pixel region between electrodes applied with a voltage. Therefore, the light-scattering property of each pixel region is so controlled that the intensity of the transmitted light or the reflected light changes in accordance with a picture signal. Consequently, one of the transmitted light and the reflected light is guided to the screen by a projection optical system.
The function of a modulating device having the fine-particle dispersion type liquid crystal layer is basically identical with that of the modulating device having the polymer dispersion type liquid crystal layer.
As a display apparatus of another type, a micro mirror device (DMD) is introduced in SID93 Digest, from page 1,012. In the micro mirror device, the angles of micro mirrors arranged in a two-dimensional matrix manner are independently changed to control the direction of reflected rays, and the light reflected in a desired direction is guided to the screen by a projection optical system. The micro mirror device also functions in the same fashion as the modulating device having the fine-particle liquid crystal layer or the polymer dispersion type liquid crystal layer in that the spatial propagation direction of the incident light rays is modulated.
FIG. 1 schematically illustrates the arrangement of a conventional projection display apparatus. In FIG. 1, a light source unit 11 is constituted by a lamp 12 as a light source and a collimator optical system 13 for collimating light from the lamp 12 into parallel rays. A scattering type modulating device 14 has a function of two-dimensionally modulating the spatial propagation direction of the incident parallel rays from the collimator optical system 13. The scattering type modulating device 14 is, e.g., a polymer dispersion type modulating device. A driver 20 drives the modulating device 14 in accordance with a picture signal. A projection optical system 16 includes an aperture stop unit 15 for extracting light rays within a certain fixed angle range from the light transmitted through the modulating device 14. The projection optical system 16 projects the extracted transmitted light onto a screen 17. In this manner, an image is displayed on the screen 17 with a light intensity distribution corresponding to the picture signal.
The contrast and the luminance of the displayed image depend upon the angle distribution of light rays constituting the exit light from the modulating device 14 and used in the display. The contrast improves as the angle distribution decreases, and the luminance improves as the angle distribution increases. That is, the contrast and the luminance of the displayed image are contrary to each other.
Jpn. Pat. Appln. KOKAI Publication No. 5-216004 or 5-188345, for example, has disclosed a technique by which the relationship between the contrast and the luminance of a displayed image is optimized in accordance with the luminance of the use environment by using an arrangement in which the aperture size of the aperture stop unit 15 for stopping down the exit light rays from the modulating device 14 is made variable.
Unfortunately, it is difficult to improve both the contrast and the luminance to respective satisfactory degrees simply by making the aperture size variable in accordance with the luminance of the use environment.